Heretofore, the sole method which enables atomization of highly viscous oil has been thought to be in the heating of air to be supplied to approximately 93.degree. C.-150.degree. C., and no other practical method has yet been found. There was an attempt to reduce the viscosity of highly viscous oil by heating the oil thereby obtaining the desired oil fog, but the result was impractical in that the amount of the oil mist was absolutely too small. When the lubricating oil is heated and maintained at high temperatures, oxidative deterioration of the oil progresses so as to impair the performance of the oil. This is another drawback in the attempt to heat the oil. Although the atomization of a predetermined amount of highly viscous oil has become possible by the presently practiced method of heating the air to be supplied, the method also has its deficiencies. One of the drawbacks is that the heating of large quantities of air requires a much larger quantity of heat than heating the oil. Also, due to the necessity of a heating chamber for the air to be supplied, the size of the apparatus itself must be enlarged. Furthermore, because the lubricated portions of the machines are cooled by the air which transports the fluid lubricant in the form of oil mist, the cooling ability of the air-oil mist unavoidably decreases due to the reduction in specific gravity of the air when the supplied air is heated, assuming that the amount of the lubricating oil is kept constant. For example, comparing the air at room temperature (25.degree. C.) with that at 120.degree. C., the former is approximately 30% superior to the latter in cooling the lubricated portions of a machine. This presents a serious problem particularly when a temperature rise of a bearing and the like must be avoided in order to improve the performance of a machine on which the aerosol-type lubrication is practiced. The drawback is especially notable with severe lubricating conditions requiring a high viscosity oil.